A battery-based power supply typically includes a protection circuit for detecting battery conditions, so as to prevent dangerous operations such as over-voltage and under-voltage during charging or discharging of the battery. Conventionally, as shown in FIG. 1, a battery package 10 for portable electronic devices includes a battery 12 and a protection circuit 13. In the protection circuit 13, MOSFETs M1 and M2 have body diodes D1 and D2, respectively, which are arranged in a back-to-back manner, and a detector 14 monitors the voltage of the battery 12 to switch the MOSFETs M1 and M2 accordingly, for controlling the currents going in to and coming out from the battery package 10. During charging operation, a charger 18 provides a charge current to charge the battery 12; during discharging operation, the battery 12 provides a discharge current to a load 16.
Under normal operation, both the MOSFETs M1 and M2 are on to allow the charge current or the discharge current to pass therethrough. Since the MOSFETs M1 and M2 have small voltage drops thereacross, neither of the body diodes D1 and D2 is conductive. However, when the detector 14 detects that an over-voltage event happens to the battery 12, the MOSFET M1 will be turned off while the MOSFET M2 will be turned on, so that the body diode D1 blocks the charge current and only the discharge current is allowed to pass through. On the other hand, when the detector 14 detects that an under-voltage event happens to the battery 12, the MOSFET M1 will be turned on while the MOSFET M2 will be turned off, so that the body diode D2 blocks the discharge current and only the charge current is allowed to pass through.
However, the conventional protection circuit 13 requires two MOSFETs M1 and M2, thereby requiring more costs and die area. Moreover, when the discharge current or the charge current flows through the body diode D1 or D2, it will produce a voltage drop across the body diode D1 or D2 and cause unnecessary power dissipation and heat generation.